The water cannon were fitted with stereos so police operating them could hear the news, the GLA said.
Photograph: Andrew Matthews/PA

Officials in Boris Johnson’s administration while he was London mayor spent nearly £1,000 fitting stereos to three water cannon that were never approved for use on the capital’s streets, it has emerged.

Johnson’s successor, Sadiq Khan, revealed that nearly £323,000 had been spent to buy, modify and maintain vehicles, which were purchased secondhand from German federal police in 2014 in anticipation of social unrest.

Greater London Authority officials said they hoped to sell on the water cannon for between £35,000 and £43,000 – more than they cost to purchase, but far less than the funds spent on them to date. However, no deal has yet been agreed.

A minimal reserve price will be set to ensure the machines, which were built in 1990, are sold for more than their scrap value, the GLA said. Any funds recouped will go towards communities and youth projects.

“It beggars belief that such a huge amount of taxpayers’ money has been wasted on paying to store these redundant machines,” Khan said. “We’ve been left in this position by the previous mayor, who rashly purchased them before he even had permission to use them, and now it’s my job to claw back as much of London taxpayers’ money as I can.

“I have spent a significant amount of time looking into how I can do this and have been left with no choice but to sell these machines through a process that charges a fee.”

A GLA source said she understood that the stereos, which included radios and CD players, had been installed to the water cannon so police officers operating the vehicles were able to listen to the news while out on the streets.

But a second official stressed that while the money for the machines’ modifications came from the GLA, it was the Metropolitan police that had decided how it was spent. The Guardian has contacted the Met for an explanation of why it decided that they required stereos with CD players.

Other costs incurred during the modification and maintenance of the machines between April 2014 – when they were bought – and April 2016 include:

£30,625.96 for conversion to UK specification and service parts

£19,376.70 for the supply and fitting of CCTV

£8,584.50 for the supply and fitting of warning equipment

£3,109.20 for the supply and fitting of 999 sirens

£1,688.04 on MOT tests

Johnson decided to buy the three Wasserweffer 9000 water cannon in 2014 after the Met requested that they be made available to address a gap in public order tactics recognised following the riots of 2011.

However, their purchase came before Theresa May, then home secretary, had approved their use – in the face of overwhelming opposition from the public. The following year, May refused to authorise their use by any police force in England and Wales, in what was seen as a snub to Johnson.

By selling the cannon, a key Khan manifesto pledge, the GLA would save almost £175,000 over the next eight years, officials claimed. They said this money would be spent on vital frontline services to help tackle the causes of crime.

Khan said: “By working with communities and returning to real neighbourhood policing, we can do far more for the safety of our city rather than relying on obsolete and illegal water cannon.

“They do not belong on the streets of London and by selling them, we’re able to put money back into helping young people affected by gang crime, and keeping Londoners safe.”